The invention relates in general to interstand tension control apparatus for a rolling mill, and more particularly to looper control apparatus in a tandem or continuous hot strip rolling mill.
The looper is a mechanism installed between any two stands, in a tandem mill installation. This mechanism provides for a desired tension on the strip as it is fed and being processed through the mill.
Although there are several loopers on a multistand tandem hot strip mill, actually one looper between each stand, the description hereinafter will refer to a single looper to illustrate any one of the loopers located between any two stands. The stand at which the strip initially enters before reaching the looper arm will be referred to as the entry stand and the stand after the looper arm will be referred to as the exit stand. The looper table projects an arm carrying a roller across the pass line between stands and it maintains under the torque of the looper motor an adequate tension on the strip once the looper roller has engaged the strip and formed the loop. Initially, the looper arm and roller are in the rest position below the pass line. The looper table is actuated as soon as the strip from the entry stand has reached and passed through the exit stand, past the looper table. In order to extend the looper arm swiftly across the pass line, the looper drive motor is so energized as to generate a looper "motor current spike" causing an "auto raise kick" of the looper mechanism.
The final position of the roller, or "height" of the looper, in operation is defined by the inclination angle of the looper arm. It depends upon the size of the loop derived from the entry stand. The interstand tension is maintained constant by the torque of the looper drive motor applied to the arm and looper roller. Such torque is controlled for a given "height", or set point, and in accordance with the corresponding loop length.
More specifically, the height of the looper table roller is regulated to a set point by a looper position regulator actuated when the strip enters the exit stand. This regulation is accomplished by the looper position regulator through control of the speed of either the entry stand or the exit stand, thereby adjusting the loop length to the looper height, e.g., the looper table angle. On the other hand, the looper drive motor torque is programmed to vary as a function of the looper height, e.g., looper table angle, so as to maintain the strip tension constant. Regulation of the looper motor drive torque is by control of the looper drive motor current.
A problem arises, however, which is due to the simultaneous operations of these two types of control. Variations of the looper motor torque build-up as the looper height varies and an unmatched anticipated height under the "auto raise kick" from the looper motor current spike, may result in hunting.
A variation in looper motor torque causes a fluctuation in the force being applied to the strip where the looper arm rollers make contact with the strip. This strip force variation is a disturbance to the looper position regulator which is trying to maintain a preset strip loop height. If the strip force disturbance causes looper oscillations, this can cause a change in the tension of the strip which, in turn, causes the strip to be rolled off-gauge, and also there can be a strip width reduction called "strip necking". Looper oscillations may also be damaging to the mill installation if the strip would move off-center and leave the stand rolls towards the mill housing after jumping out of the strip guides.
One attempt of preventing this from occurring has been to remove the "auto raise kick" feature, that is the initial looper motor current spike, and to operate without torque adjustment to the table angle, e.g., the looper height. Thus, the system would operate at constant motor torque. Unfortunately, this is not an acceptable practice because at low looper height too much tension is exerted upon the strip, or loop, whereas at high looper operating height, not enough tension is applied to the strip to keep a tight strip between the stands. It appears that regulation of the looper torque as a function of the looper height cannot be dispensed with if a tight strip is to be maintained during looper transients.
In the absence of the "auto raise kick" feature, the looper roll is too slow to raise itself and make contact with the strip following energization of the looper regulator. It is important that the strip loop be formed as soon as possible if delivery strip gauge and width is not to vary from the set point.